surface pretreatment by phosphate conversion coatings – a review

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Surface pretreatment by phosphate conversion coatings – a review coating formation to a great extent. The most commonly used methods of deposition namely the dip and spray process, although have received considerable attention, it is believed that the dip process is most likely to produce an equiaxed structure which is consider to be beneficial from the corrosion protection point of view. The influence of permanent magnetic field during phosphating is studied by Bikulcius et al. [303]. According to them, when exposed to perpendicular magnetic field, the zinc phosphate coating results in larger crystallites with lower corrosion resistance whereas the effect of parallel magnetic field is insignificant. However, the application of a magnetic field either in perpendicular or parallel mode results in a fine-grained, uniform and more compact zinc calcium phosphate coating. Electrochemical method of phosphating always yields a heavy coating weight [148,304]. In recent years, studies have been attempted in electrochemical phosphating to produce phosphate coatings richer in a particular phase by selecting an appropriate applied potential [147]. The success of this kind of phosphating methodology has been restricted by the difficulties encounter in adding ‘electrics’ to the existing process plants. Mechanical vibration of steel during phosphating is considered as a method for obtaining a fine grained phosphate coating. However, the amount of coating formed is found to be inversely proportional to the frequency of vibration [305]. Ultrasonically induced cavitation produces a great number of active centres, which results in a high rate of nucleation. This leads to a uniform fine-grained phosphate coating with low porosity [307,308]. 3.8. Processing problems and remedial measures Maintenance of the bath parameters and operating conditions at an optimum level is a major and complicated process, especially in continuous and largescale phosphating. In actual practice, the finishers have come across several situations that the bath is not working properly. These problems viz., overheating of the bath, creation of an excess acidity at the metal/solution interface especially in the case of cold phosphating, change in acidity of the bath due to carryover of the alkaline solution used for degreasing, the local increase in acidity due to the scaling of the heating coils and so on, were discussed elsewhere [129, 302]. Moreover, baths that have been used for a long time are rendered ineffective due to the formation of sludge of ferric phos- 153 phate. Sludging occurs to a greater extent in unaccelerated and highly acidic baths and has to be removed periodically to assure proper bath operation [18,19]. In order to maintain bath parameters and to enhance bath life, make-up solutions and solids are usually required. These additives are so formulated that they can be handled easily and are inexpensive. 3.9. Defects in coatings and remedies The most commonly encountered defects in phosphated parts are low corrosion resistance, stained coating with variable corrosion resistance and coatings covered with a loose white powdery deposit. The possible cause for these defects are insufficient care in degreasing and cleaning, incorrect acid coefficient, incorrect solution composition, use of incorrect operating conditions, incorrect maintenance of chemicals, excessive sludge formation and faults in after-treatment. These causes, either singly or in combination, lead to these defects. Low corrosion resistance of coatings may be a result of incorrect acid coefficient, incorrect bath parameters and operating conditions, presence of certain metallic impurities like aluminium, antimony, tin and lead compounds and the presence of chloride ions in the bath. Improper sealing and abnormally thin coatings may also lead to inferior corrosion resistance. Stained coatings may be formed due to improper cleaning and degreasing. Incorrect distribution of articles in the phosphating bath and wrong ratio of work surface to solution volume may be other causes. Over heating of the phosphating bath, make-up during processing, heavy sludging and sludge suspended in the bath leads to the formation of coatings with a loose powdery deposit. Proper cleaning and degreasing, correct maintenance of work surface to solution volume ratio, control of bath parameters and operating conditions within the strict limits, avoidance of overheating and excessive sludge formation will yield coatings of consistent good quality and corrosion resistance. 3.10. Characterization of phosphate coatings A variety of instrumental methods, which include, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), X-ray fluorescence (XRF), extended X-ray absorption fine structure (EXFAS), Fourier transform
154 T.S.N. Sankara Narayanan infra red spectroscopy (FTIR), Raman spectroscopy, differential thermal analysis (DTA), differential scanning calorimetry (DSC), secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM), glow discharge optical emission spectrometry (GDOES), quartz crystal impedance system (QCIS), conversion electron Mossbauer spectrometry (CEMS), acoustic emission (AE) testing, etc. were used to characterize phosphate coatings [308-340]. SEM is the most commonly and widely used technique for characterizing phosphate coatings. It is used to determine the morphology and crystal size of phosphate coatings. SEM serves as an effective tool to study the nucleation and growth of the phosphate coatings and makes evident of the fact that the initial growth of the phosphate coating is kinetically controlled and at a latter stage it tends towards mass transport control [322]. SEM also substantiates the fact that preconditioning the substrate before phosphating enables the formation of a fine-grained and adherent phosphate coating. EPMA is used in the determination of the porosity of phosphate coatings in which the number of copper spots deposited in the pores following immersion in copper sulphate solution (pH 5.0) [323]. XRD is primarily used to detect the phase constituents present in phosphate coatings – the phosphophyllite and hopeite phases in zinc phosphate coating. Though the manganese and nickel modification of zinc phosphate coating reveals only the presence of hopeite phase, there observed to be significant variations in the orientations of the hopeite crystals as evidenced by the relative intensities of the H(311), H(241) and H(220), H (040) peaks. XRD is also used to characterize phosphate coatings in terms their ‘P’ ratio. Van Ooij et al. [324] applied high resolution Auger electron spectroscopy (AES) in combination with energy dispersive X-ray spectrometry (EDX) to study the nature of chromium post passivation treatment. According to them, Cr(III) was not detected at the metal/phosphate boundry but on the surface of phosphate crystals and that the Fe/Zn ratio was increased in the surface and subsurface layers. XPS is used to detect the nature of various species in phosphate coating. The presence of fatty acid-like contaminants on the metal substrates can be identified from the C 1s spectra. Similarly, the Zn 2p 3/2 and P 2p spectra enable identification of Zn 3 (PO 4 ) 2 . Besides, the formation of ZnO or Zn(OH) 2 and NaHPO 4 could be identified from the Zn 2p 3/2 and P 2p spectra, respectively. The Fe 2p 3/2 will give an idea about the presence of Fe 2 O 3 and FePO 4 in the coating [187]. Cu 2p spectra obtained from the sample phosphated using a solution containing 1 ppm of Cu 2+ ions show two components at binding energies 933.6 eV and 932.2 eV [325]. The former signifies Cu 2+ , although the latter could arise from either Cu + or Cu metal. All evidence supports the Cu deposition occurring during the phosphating process. ESR is used to confirm the modified structure of hopeite films formed on the surface of pre-treated steel sheets. The zinc phosphate film formed from the bath that does not contain manganese or nickel ions exhibits no ESR signal. This is because ESR could only detect paramagnetic transition metal ions with an unpaired electron whereas in zinc phosphate coating the ten electron spins of zinc (II) metal ions (3d 10 ) in the ‘d’ orbitals are paired with one another. However, ESR detects the manganese and nickel components in the modified zinc phosphate coating and proves that manganese and nickel exist as Mn(II) and Ni(II) in these coatings [316]. XRF is used to determine the nature of nickel in nickel modified zinc phosphate coating. The XRF spectrum obtained for metallic nickel is compared with that of the modified zinc phosphate coating. The Ni L α peak of metallic nickel occurs at 34.18° whereas the Ni L α peak of the nickel component of modified phosphate coatings occurs at 34.06°, indicating that the nickel component in the films is not in the metallic state [316]. EXFAS was used to assess the crystal structure of manganese modified zinc phosphate coating [318]. The radial distributions of the first neighbouring atoms appeared at a distance of 0.146 nm for unmodified hopeite and at 0.144 nm for manganese modified hopeite. This decrease is due to the disorderness of modified zinc phosphate coating following the introduction of manganese. EXFAS study also confirms that the manganese component is substituted for zinc component in the octahedral structure. SIMS was used to detect the presence of titanium, which are adsorbed on to the metal surface and act as sites for crystal nucleation [313]. The crystal structure of hopeite and phosphophyllite are hydrated. So once heated, dehydration reactions are expected to occur. Differential thermal analysis (DTA) performed on phosphate coatings shows that there is a clear disparity in the dehydration process of these two phases as evidenced by a 50 °C difference in temperature in the endothermic peaks corresponding to hopeite and phosphophyllite phases. In the case of nickel and
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surface pretreatment by phosphate conversion coatings – a review

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